A trajectory guide can be mounted onto a subject's skull about a desired skull entry portal, such as for guiding an instrument through the skull entry portal and toward a desired path into the subject's brain.
Carol U.S. Pat. No. 4,805,615 provides an example of a ball-and-socket trajectory guide.
Solar et al. U.S. Pat. No. 7,497,863 provides an example of a rotate-and-sweep trajectory guide.
Solar et al. U.S. Pat. No. 8,747,419 provides an example of a ball-and-socket trajectory guide in which a central pivot point of the ball is located below a surface of the skull when the base is affixed to the skull of a subject.